How does shot peening improve fatigue life and where is it used?

Shot peening introduces beneficial compressive residual stress in surface layers. Process: High-velocity spherical media (steel, glass, ceramic shot) impact surface, Plastic deformation creates compressive stress layer (0.1-0.5mm deep), Surface hardening also occurs. Benefits: Compressive stress opposes tensile loading, Retards fatigue crack initiation, Can double or triple fatigue life, and Reduces stress corrosion cracking susceptibility. Applications: Landing gear (high-strength steel), Fastener holes (aluminum and titanium), Gear teeth, Spring elements, and Turbine components. Process control: Almen strips measure intensity, Coverage verified visually or UV, Saturation curve defines required exposure. Variations: Laser shock peening - Deeper compression, more controlled; Ultrasonic peening - Less distortion; and Dual peening - Two media sizes for surface finish and depth. Specifications define intensity, coverage, and media requirements. Post-peening operations (grinding, etching) must not remove beneficial layer. Shot peening is standard practice for fatigue-critical components.